implement notification/prompt to user to backup their wallet

[LaurenWags]

Description

We should prompt user to backup their Brave Wallet if they have not already done so. This was implemented on browser-laptop with brave/browser-laptop#13425

Steps to Reproduce

1. User enables Brave Rewards.
2. User does not backup wallet after set time (perhaps 1 week?).
3. Prompt/Notify user to backup their wallet.

Additional Information

Test Plan for browser-laptop (muon) issue is in brave/browser-laptop#13504

[LaurenWags]

Verified passed with

Brave	0.60.23 Chromium: 72.0.3626.96 (Official Build) beta(64-bit)
Revision	84098ee7ef8622a9defc2ef043cd8930b617b10e-refs/branch-heads/3626@{#836}
OS	Mac OS X

• Verified the below (tests adapted from Add rewards notification to backup wallet brave-core#1044)
  Test 1
• Launch with clean profile and - -rewards=staging=true
• Enable Rewards, don't add any funds to wallet
• Close Brave
• Use your favorite code editor to modify the Preferences file in your profile folder to add backup_notification_frequency and backup_notification_interval to the brave.rewards section. (I used "backup_notification_frequency":"50000000","backup_notification_interval":"50000000")
• Launch Brave (don’t forget staging flag if used) and wait.
• Rewards icon does not badge
• brave.rewards.backup_succeeded was not added to Preferences file for profile

Test 2

• Launch with clean profile and - -rewards=staging=true
• Enable Rewards, add funds to wallet
• Close Brave
• Use your favorite code editor to modify the Preferences file in your profile folder to add backup_notification_frequency and backup_notification_interval to the brave.rewards section. (I used "backup_notification_frequency":"50000000","backup_notification_interval":"50000000")
• Launch Brave (don’t forget staging flag if used) and wait.
• Verify backup notification from PR is displayed.
• Clear notification
• Bring up wallet properties dialog and select Copy
• Close Brave.
• brave.rewards.backup_succeeded is set to true in Preferences file for profile
• Launch Brave (don’t forget staging flag if used) and wait the appropriate amount of microseconds.
• Rewards icon does not rebadge
  Repeat this test for Print and Save to File options on wallet properties dialog.

Test 3

• Launch with clean profile and - -rewards=staging=true
• Enable Rewards, add funds to wallet
• Close Brave
• Use your favorite code editor to modify the Preferences file in your profile folder to add backup_notification_frequency and backup_notification_interval to the brave.rewards section.
• Launch Brave (don’t forget staging flag if used) and wait.
• Verify backup notification from PR is displayed.
• Clear notification
• Don’t take any backup actions.
• Close Brave.
• brave.rewards.backup_succeeded was not added to Preferences file for profile
• Launch Brave (don’t forget staging flag if used) and wait for the two items you added.
• Rewards icon does rebadge
  

Verification passed on

Brave	0.60.28 Chromium: 72.0.3626.96 (Official Build) beta (64-bit)
Revision	84098ee7ef8622a9defc2ef043cd8930b617b10e-refs/branch-heads/3626@{#836}
OS	Windows 7

Used above test plan. Thanks @LaurenWags !

Verification PASSED on Mint 19.1 x64 using the following build:

Brave	0.60.29 Chromium: 72.0.3626.96 (Official Build) beta (64-bit)
Revision	84098ee7ef8622a9defc2ef043cd8930b617b10e-refs/branch-heads/3626@{#836}
OS	Linux

• Went through the cases @LaurenWags provided without any issues